The invention relates to variable speed electrical devices and, more particularly, to methods of and means for infinitely varying the speed of rotation of the drive shaft of an alternating current rotary machine. Specifically, this invention concerns improvement of the structure and operation of a variable speed, alternating current motor unit of the type which is disclosed in my patent for "Method of and Means for Varying the Speed of Alternating Current Motors", U.S. Pat. No. 2,787,747, which issued Apr. 2, 1957.
The variable speed, alternating current motor unit which is disclosed in my U.S. Pat. No. 2,787,747 includes a plural induction motor arrangement wherein two induction motors are arranged end-to-end in a housing. The stator elements are rotatably mounted in the housing and are mechanically linked end-to-end by a gear train so that rotation of one stator element opposes rotation of the other stator element so as to buck one another when both of the induction motors are energized. Consequently, torque is induced in the rotor elements which are either (a) connected to a common output shaft which is rotatably mounted in the housing so as to apply additive torque to drive a load or (b) connected to separate output shafts which are rotatably mounted in the housing so as to apply equal torque to drive separate loads.
The variable speed, alternating current motor unit which is disclosed in my U.S. Pat. No. 2,787,747 further includes a speed control circuit with a saturable core reactor. The voltage difference between a rectified tachometer voltage generated by the unit and a reference voltage established by a speed setting potentiometer controls the magnetic flux density in the core of the saturable core reactor to regulate the amount of power to one of the induction motors. This controls the extent to which the stator elements buck one another and, hence, regulates the speed of the unit.
The variable speed, alternating current motor unit which is disclosed in my U.S. Pat. No. 2,787,747 has proved quite satisfactory and has performed capably in many industrial and other types of applications. Nevertheless, there are some applications in which the structure and operation of the unit have made use of the unit impractical or infeasible.
The primary limitation is the size of the unit which is disclosed in my U.S. Pat. No. 2,787,747. Specifically, since the induction motors are arranged end-to-end, the unit is of considerable length. The length is further increased by the configuration of axially aligned bevel and idler gears in a separate housing compartment to mechanically link the stator elements so as to buck one another. Another limitation is the weight of the unit. That is, the mechanically linked induction motors in the end-to-end arrangement are contained in a housing with a plurality of compartments which are separated by heavy wall plates that support antifriction bearings. The need for additional compartments which are also separated by heavy wall plates that support antifrictional bearings are indicated if speed reduction gears are included between a rotor element and an associated output shaft. Consequently, the amount of material which comprises the housing for the mechanically linked induction motors in the end-to-end arrangement renders the weight of the unit substantial, even if high strength, lightweight materials are utilized in construction.
As a result of the considerable length and weight of the unit which is disclosed in my U.S. Pat. No. 2,787,747, employment as a drive system is limited to applications with relatively heavy machine equipment, such as milling or printing machines, large vehicles, such as buses, etc. Employment as a drive system in light machine equipment, such as drill presses, small vehicles, such as economy-size cars, etc. has not been possible due to the considerable length and weight of the unit.
Furthermore, the speed control circuit which is disclosed in my U.S. Pat. No. 2,787,747 has a low response primarily due to the characteristics of the saturable core reactor. The magnetic flux density in the saturable core reactor changes relatively slowly which introduces a speed control lag. The resultant speed control lag detracts from the precision with which the speed of the unit is regulated which in turn reflects in the degree of smoothness of operation of the unit that is achievable as the unit drives a load. There is at the present time a need for precise speed control of electrical motors in more sophisticated industrial equipment and, also, in the application of electrical motors in vehicle propulsion.
The primary object of my invention is to provide a variable speed, alternating current motor unit which produces a high torque but is designed and constructed so as to produce a significant reduction in both the size and weight of the unit.
Another object of my invention is to improve the speed control circuit for a variable speed, alternating current motor unit by employing solid state components that not only reduce the size and weight of the circuit but also permit the use of modular construction to reduce manufacturing and maintenance costs.
Another object of my invention is to provide for the utilization of manual control devices and mechanical, photo-electric, tachometer or similar speed monitoring devices where the speed monitoring signal is less than two watts.
Another object of my invention is to provide a variable speed, alternating current motor unit so that the energy required to control speed instead of being dissipated in heat loss is reflexed back into the unit resulting in higher efficiency at all speed levels.
Another object of my invention is to provide a variable speed, alternating current motor unit that may be employed in a vehicle propulsion system which is accomplished by placing the vehicle's propulsion wheels on shaft extensions.
A further object of my invention is to provide a variable speed, alternating current motor unit which acts as a differential due to its characteristics to slip a few rotations without harm to the unit.
Another object of my invention is to provide a variable speed, alternating current motor unit with windings adapted for use on high frequency alternating currents with the high frequency power to be generated in a high frequency alternator driven by a direct-connected gas turbine to produce a nonpolluting vehicle propulsion system.
A further object of my invention is to provide for a decelerating force to be applied to a vehicle in motion when the throttle is closed, thus overcoming the lack of compression decelerating force in a vehicle propelled by a turbo-electric system thereby preventing the vehicle from free-wheeling.
A further object of my invention is to provide a speed control that maintains a constant desired speed.
Another object of my invention is to provide a brake system which is not subject to glazing or burning of the braking surfaces, thereby avoiding fading of the braking effort.
Another object of my invention is to provide for braking of a vehicle by employment of eddy currents created by the electrical energy supplied by a turbo-electric system which propels the vehicle.
Another object of my invention is to provide a locked-up differential.
Another object of my invention is to provide a circuit for a speed control which is applicable to either industrial or vehicular use in that the modular construction simplifies construction and maintenance.